Method of heat transfer between a metallic or non-metallic item and a heat transfer fluid

ABSTRACT

A method of heat transfer between a metallic or non-metallic item and a heat transfer fluid is provided. The method includes a fluid medium and nanoparticles. A thickness/lateral size ratio of the nanoparticles is below 0.00044. The nanoparticles do not include carbon nanotubes.

The present invention relates to a method of heat transfer between ametallic or non-metallic item and a heat transfer fluid comprising afluid medium and specific nanoparticles having a specificthickness/lateral size ratio. In particular, it is well suited forsteel, aluminum, stainless steel, copper, iron, copper alloys, titanium,cobalt, metal composite, nickel Industries or non-metallic industriessuch as plastics.

BACKGROUND

With a view of saving energy consumption, it is possible to improve theperformance of heat exchangers systems and to introduce various heattransfer enhancement techniques. Some techniques have focused onelectric or magnetic field application. Although an improvement inenergy efficiency is possible from such points of view, an improvementcan also be realized concerning the heat transfer fluid. Usually, fluidssuch as water, engine oil, ethylene glycol, etc. are used as heattransfer fluid. However, they have poor heat transfer performance andtherefore high compactness and effectiveness of heat transfer systemsare necessary to achieve required heat transfer. Among the efforts forenhancement of heat transfer, the application of additives to liquids ismore noticeable.

For example, a surfactant such as LEVENOL C-421 which is polyoxyethylenemono- and di-glycerides, can be added into water for improving the heattransfer coefficient or at least the thermal conductivity. However,although the conductivity enhances in some cases, the presence of thesurfactant results in the formation of foam. The presence of foam is ahuge problem since it is really difficult to remove it, in particular inindustrial scale. Moreover, the presence of a surfactant increases thecorrosion of the heat transfer system, specially the pipe wherein theheat transfer fluid flows. Finally, scale can be formed particularly inthe heat transfer system.

Recent investigations in nanotechnology have allowed the development ofa new category of heat transfer fluid comprising nanoparticles. Suchfluids also called “Nanofluid” are liquid suspension containingparticles having at least one dimension below 100 nm. These heattransfer fluids have usually an increased heat transfer coefficient.

The patent application US2014/0312263 discloses a heat transfer fluidcomprising a fluid medium and an oxidized form of a material selectedfrom the group of multilayer graphene nanoplatelets. It also discloses amethod for manufacturing such fluid. The patent application describesthat the oxidation of the multilayered graphene nano-platelets (GnPs)converts sp² graphite layers on the surface into OH⁻, COO⁻ and COgroups. These groups create sufficient electrostatic charge at thenanoplatelet surface that keep the particles separated from each otherdue to repulsion and prevents particle agglomeration and settling. Thus,a good stability of graphitic nanofluids in a water or ethyleneglycol/water base fluid mixtures can be achieved and therefore a gooddispersion.

It also discloses that suspensions with unmodified GnPs settle within afew hours. Suspensions stabilized with cationic or anionic surfactantsshow improvement in stability; however thermal conductivity of thosesuspensions is below the base fluid due to very low thermal conductivityof organic molecules compared to water. Thus, organic surfactants aredetrimental for the thermal conductivity for the thermal conductivity ofwater based suspensions. Therefore, the use of non-surfactant approachto stabilizing dispersions of nanoparticles involves the oxidation ofGnP, to clearly separate GnPs to individual nanoplatelets.

Finally, it discloses that oxidation of GnPs reduces the thermalconductivity enhancements in all tested grades. The ratio of heattransfer coefficients (h_(nf)/h₀) for the nanofluid (h_(nf)) and thebase fluid (h₀), calculated for different temperatures, shows that theinclusion of graphitic nanoparticles in ethylene glycol/H₂O coolant canprovide 75-90% improvement in heat transfer rates when used in laminarflow regime. Heat transfer coefficients in the turbulent flow regimeshow 30-40% improvement in heat transfer compared to the base fluid.

However, the oxidation or functionalization of GnPs necessitates anadditional step in the process for the manufacture of the heat transferfluid using strong acids, for example a mixture of concentrated sulfuricand nitric acids as in US2014/0312263. In industrial scale, thisoxidation reaction produces waste products being difficult to manage.Additionally, this heat transfer fluid does not reach very highperformance. For example, in steel making industry, during the coolingprocess in a hot rolling process, the run-out table cools the steelstrip from approximately 800-950° C. at the entrance to 450-600° C. atthe exit. Thus, for some steel grades, a heat transfer fluid having highheat transfer coefficient is needed.

SUMMARY OF THE INVENTION

An object of the present invention provides an easy to implement methodof heat transfer between a metallic or non-metallic item and a heattransfer fluid wherein the heat transfer fluid has a high heat transfercoefficient. The present invention provides a method of heat transferbetween a metallic or non-metallic item and a heat transfer fluidcomprising a fluid medium and nanoparticles wherein thethickness/lateral size ratio of such nanoparticles is below 0.00044 andwherein nanoparticles do not comprise carbon nanotubes.

The invention also provides a method for the manufacture of a heattransfer fluid comprising A., the provision of nanoparticles accordingto anyone of claims 1 to 20, B., the provision of a fluid medium, C.,the adjustment of the nanoparticle concentration in order to achievepercolation and D., the mixing of the nanoparticles with the fluidmedium.

The invention also provides a heat transfer fluid.

The following terms are defined:

heat transfer fluid comprising nanoparticles (so-called Nanofluid) meansa liquid suspension containing particles having at least one dimensionbelow 100 nm,

laminar flow means a flow with a Reynolds number below a critical valueof approximately 2300,

turbulent flow means a flow with a Reynolds number larger than acritical value of about 4000,

percolation threshold concentration is the concentration ofnanoparticles above which they are connected forming a long-rangenetwork. For heat transfer applications, it is suitable that suchnetwork connects the hottest part, i.e. the part where the heat startsto flow, of the fluid and the coldest part of the fluid, i.e. the onewhere the heat is evacuated. In other words, below the Percolationthreshold concentration, nanoparticles are not connected. When thePercolation threshold concentration is obtained, the network formed withnanoparticles, having higher thermal conductivity than the fluid medium,allows the heat carriers to take a path with much less thermalresistance, thus enhancing the thermal conductivity of the fluid, andtherefore the heat transfer coefficient,

vol. % means percentage by volume,

wt. % means percentage by weight,

graphite nanoplatelets means a multilayered system of graphene sheetshaving a thickness around between 5 and 20 nm,

few layers graphene means a multilayered system of graphene sheetshaving a thickness between 1 and 5 nm and

graphene means a single-atom-thick sheet of hexagonally arranged, bondedcarbon atoms, presenting usually a thickness below 1 nm.

Other characteristics and advantages of the invention will becomeapparent from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate the invention, various embodiments and trials ofnon-limiting examples will be described, particularly with reference tothe following Figures:

FIG. 1 illustrates an example of one nanoplatelet according to thepresent invention.

FIG. 2 illustrates an example of multilayered nanoplatelets according tothe present invention.

FIG. 3 illustrates an example of spherical nanoparticle according to thepresent invention.

FIG. 4 illustrates an example of elliptical nanoparticle according tothe present invention.

DETAILED DESCRIPTION

The invention relates to a method of heat transfer between a metallic ornon-metallic item and a heat transfer fluid comprising a fluid mediumand nanoparticles wherein the thickness/lateral size ratio is below0.00044 and wherein nanoparticles do not comprise carbon nanotubes.

Without willing to be bound by any theory, it seems that when the heattransfer fluid comprising nanoparticles having a thickness/lateral sizeratio below 0.00044, the Percolation threshold concentration decreases.Consequently, fewer bridges are formed above percolation thresholdconcentration resulting in a decrease of viscosity. In addition, it ispossible to obtain high thermal conductivity and therefore high heattransfer coefficient with low nanoparticles concentration especially inlaminar flow.

According to the invention, the flow of the heat transfer fluid can bein a laminar or turbulent flow regime. In a laminar flow regime, theheat transfer coefficient is proportional to the thermal conductivity.On the contrary, in turbulent flow regime, the heat transfer coefficientdepends on a set of thermo-physical properties such as viscosity.

Preferably, the heat transfer fluid comprises nanoparticles having athickness/lateral size ratio below 0.00043, advantageously between0.00010 and 0.00040, more preferably, between 0.00015 and 0.00035 orbetween 0.00020 and 0.00030.

Advantageously, the thickness of nanoparticles is between 1 and 99.99nm, preferably between 5 to 50 nm and more preferably between 5 to 15nm.

Preferably, the lateral size of the nanoparticle is between 26 and 50μm, advantageously, between 35 and 45 μm.

Preferably, the nanoparticle concentration is between 0.01 wt. % and 12wt. %, advantageously between 2 and 8 wt. % and more preferably between4 and 7 wt. %.

For example, the nanoparticle can be spherical, elliptical ornanoplatelets.

FIG. 1 illustrates an example of one nanoplatelet that can be used inthe heat transfer fluid of the present invention. In this example, thelateral size means the highest length of the nanoplatelet through the Xaxis of FIG. 1 and the thickness means the height of the nanoplateletthrough the Z axis. The width of the nanoplatelet is illustrated throughthe Y axis.

FIG. 2 illustrates an example of multilayered nanoplatelets that can beused in the heat transfer fluid of the present invention. In thisexample, the lateral size means the highest length of the nanoplateletsthrough the X axis and the thickness means the total height of all thestacked nanoplatelets through the Z axis. The width of the nanoplateletis illustrated through the Y axis.

FIG. 3 illustrates an example of spherical nanoparticle that can be usedin the heat transfer fluid of the present invention. In this example,the lateral size means the diameter of the nanoparticle and thethickness means the height of the nanoparticle.

FIG. 4 illustrates an example of elliptical nanoparticle that can beused in the heat transfer fluid of the present invention. In thisexample, the lateral size means highest length of the nanoparticle andthe thickness means the height of the nanoparticle.

The lateral size and the thickness of the nanoparticle can be measuredby Scanning Electron Microscopy (SEM), Transmission Electron Microscopy(TEM) and Atomic Forces Microscopy (AFM).

In a preferred embodiment, the heat transfer fluid comprisesnanoparticles being multilayered nanoplatelets. Indeed, without willingto be bound by any theory, it seems that to obtain nanoplateletsmorphology, nanoparticles should have a multilayer structure with weakinteraction between layers, i.e. Van der Waals, hydrogen bond,mechanical bond, halogen bond, pi stacking, cation/anion-pi bonds,intercalation, salt bridges and polar-pi. This weak bonding togetherwith a good thermal conductivity of the nanoplatelets raises thepossibility of improving heat transfer coefficient of a fluid.

Preferably, nanoparticles are chosen from graphite nanoplatelets,graphene, few layers graphene, TiO₂, ZnO₂, ZnO, Boron-nitride, copper,silica, montmorillonite, zeolite clipnoptilolite, wollastonite, mica,zeolite 4A, Al₂O₃, silicate, pumice and calcium oxide.

In a preferred embodiment, the heat transfer fluid further comprises adispersing agent. The dispersing agent can be a non-surface activepolymer, a surfactant or a mixture thereof. The surfactant can becationic, anionic, amphoteric or non-ionic.

For example, the dispersant agent can be polyvinnylpyrrolidone,polysaccharides, sulphated polysaccharides, linear alkylbenzenesulfonates, lignin sulfonates, di-alkyl sulfosuccinates, quaternaryammonium compounds, sodium stearate or a mixture thereof.

Preferably, the nanoparticles concentration/dispersing agentconcentration ratio in weight is between 3 and 18. More preferably, thenanoparticles concentration/dispersing agent concentration ratio isbetween 4 and 15, advantageously between 4 and 8 and preferably beingbetween 4 and 6.

Without willing to be bound by any theory, it seems that when the aboveratio is controlled and the Percolation threshold concentration reached,the heat transfer fluid according to the invention allows for a higherthermal conductivity and therefore a higher heat transfer coefficient.Indeed, the dispersing agent would be able to avoid deposition andagglomeration of nanoparticles. For instance, if the dispersing agent isa surfactant, the nanoparticle would be enclosed by a micelle consistingin a core of hydrophobic molecules and a shell of hydrophilic molecules.Such micelle structure allows dispersing nanoparticles within the fluid.However to obtain percolation, in other words the formation of thelong-range network formed by the nanoparticles, the degree of dispersionof nanoparticles has to be limited.

Preferably, the heat transfer fluid comprises a fluid medium chosen fromwater, ethylene glycol, ethanol, oil, methanol, silicone, propyleneglycol, alkylated aromatics, liquid Ga, liquid In, liquid Sn, potassiumformate and a mixture thereof. Gallium, Indium and Tin can be used asheat transfer fluid, in particular for the cooling of a metallic item.Indeed, the melting point of gallium is of 30° C., the one of indium is157° C. and the one of tin is of 232° C. For example, they can be usedto cool down computer chips or laboratory equipments such as neutronsources.

According to the invention, the heat transfer method is between ametallic or non-metallic item and the heat transfer fluid. Preferably,the metallic item, being for example a metallic substrate, is made ofaluminum, steel, stainless steel, copper, iron, copper alloys, titanium,cobalt, metal composite, nickel and the non-metallic is made ofplastics.

In the prior art, the heat transferring using water as fluid medium canusually be realized by 2 different modes. The first mode is called“non-contact water” which means that water is kept in a circuit withoutbeing shot towards the object, off-gases or fluids to cool or to heat.This mode uses indirect cooling or heating systems or non-contactcooling or heating systems, in particular through heat exchangers. Thesecond mode is called “contact water” which means that water is used tocool or heat an object by being in direct contact with it.

According to one preferred embodiment of the invention, the item, beingmetallic, is a heat exchanger and the heat transfer is realized with afluid being inside the heat exchanger.

In particular, in the steel making industry, the heat transfer using aheat exchanger can be implemented in coke oven gas treatment, blastfurnaces, basic oxygen furnaces, electric arc furnaces, continuouscasting, hot-rolling operations, cold-rolling operations, boilers,annealing furnaces and coating, pickling or sinter lines. The cooling insuch processes is needed for maintain performance of processingequipment.

According to one preferred embodiment of the invention, the item is ametallic substrate and the heat transfer fluid is directly in contactwith it. In this case, the heat transfer can be realized by jetimpingement cooling, pool boiling, spray cooling or micro-channelcooling.

For example, in the steel making industry, the heat transfer by contactwater cooling can be implemented:

in sprays chambers of continuous casters and hot rolling process such asthe cooling process on the run-out table,

In coke ovens for gas treatment and quenching of coke,

during the slag quenching in blast furnaces, basic oxygen furnaces andelectric arc furnaces.

The heat transfer fluid is preferably manufactured by the followingsteps:

-   -   A. the provision of nanoparticles according to the present        invention,    -   B. the provision of a fluid medium,    -   C. the adjustment of the nanoparticle concentration in order to        achieve percolation and    -   D. the mixing of the nanoparticles with the fluid medium.

The heat transfer fluid of the present invention has high heat transfercoefficient and preferably a good dispersion.

The invention will now be explained in trials carried out forinformation only. They are not limiting.

EXAMPLES Heat Transfer Coefficient-Laminar Flow

Example 1

Trials 1 to 3 were prepared by mixing graphite nanoplatelets having athickness/lateral size ratio of 0.00025, 0.001 and 0.005 with water.

For each trial, the thermal conductivity of the samples has beenmeasured employing a DTC-25 thermal conductivity meter. The thermalconductivity enhancement was calculated with respect to the conductivityof water at room temperature, i.e. 20° C., the conductivity of water insuch conditions being 0.67 W/mK. Trials 4 to 6 are respectively samplesA-GnP, B-GnP and C-GnP having functionalized nanoparticles of the Patentapplication US2014/0312263. The thickness/lateral size ratio isrespectively of 0.001-0.009, 0.0005-0.008, and 0.00044-0.003.

For all the trials, in laminar flow, the heat transfer enhancement isproportional to the enhancement of thermal conductivity, so nocalculation is needed to have the heat transfer enhancement in %.

Heat Nanoparticles transfer concentration Thickness/lateral enhancementTrials Samples (wt. %) size ratio (%) 1* 1 5 0.00025 203 2 2 5 0.001 313 3 5 0.005 10 4 4 5 0.001-0.05 6 5 5 5 0.0005-0.008 75 6 6 50.00044-0.005  85 *according to the present invention.

Trial 1 has a high heat transfer enhancement when compared to Trials 2to 6.

-   -   Example 2

Trials 8 and 9 were prepared by mixing graphite nanoplatelets having athickness/lateral size ratio of 0.00025 and 1 wt. % ofpolyvinnylpyrrolidone as dispersing agent with water.

The thermal conductivity of the samples has been measured employing aDTC-25 thermal conductivity meter. The thermal conductivity enhancementwas calculated with respect the conductivity of water. In laminar flow,the heat transfer enhancement is proportional to the enhancement ofthermal conductivity, so no calculation is needed to have the heattransfer enhancement in %.

Heat transfer Nanoparticles Dispersing C_(nanoP)/ Thickness/ enhance-concentration agent C_(disp) lateral ment Trials Samples (wt. %) (wt. %)ratio size ratio (%) 7* 1  5 — — 0.00025 203 8* 7  7 1  7 0.00025 286 9*8 10 1 10 0.00025 384 *according to the present invention

Trials 8 and 9 having a dispersing agent have a higher heat transferenhancement than Trial 7 without a dispersing agent.

-   -   Example 3

The cooling performance of Trials 1 to 9 and Trial 10, consisting ofwater, was calculated thanks to a modeling software. In this test, asteel slab having a density of 7854 kg/m³ was cooled in laminar flowduring 13 seconds. The length was of 5 meter, the width of 1 meter andthe slab thickness was of 10 mm.

The initial temperature of the slab was of 968° C. The following tableshows the cooling rate by using each Trial:

Trials Cooling rate (° C./s)  1* 36.9 2 26.1 3 22.9 4 22.3 5 32.7 6 34.2 7* 36.8  8* 46.9  9* 54.9 10  21.4 *according to the present invention

Trials 1, 7, 8 and 9 have a higher cooling rate than Trials 2 to 6 andTrial 10.

What is claimed is:
 1. A method of heat transfer between a metallic ornon-metallic item comprising the step of: transferring heat between ametallic or non-metallic item and a heat transfer fluid including afluid medium and nanoparticles, a lateral size of the nanoparticlesbeing between 26 and 50 μm, a thickness/lateral size ratio of thenanoparticles being more than or equal to 0.00015 and less than 0.00044,wherein the nanoparticles do not include carbon nanotubes.
 2. The methodaccording to claim 1, wherein the thickness/lateral size ratio is morethan or equal to 0.00015 and less than 0.00043.
 3. The method accordingto claim 2, wherein the thickness/lateral size ratio is from 0.00015 to0.00040.
 4. The method according to claim 3, wherein thethickness/lateral size ratio is from 0.00015 to 0.00035.
 5. The methodaccording to claim 4, wherein the thickness/lateral size ratio is from0.00020 to 0.00030.
 6. The method according to claim 1, wherein athickness of the nanoparticles is from 1 to 99.99 nm.
 7. The methodaccording to claim 6, wherein the thickness of the nanoparticles is from5 to 50 nm.
 8. The method according to claim 7, wherein the thickness ofthe nanoparticles is from 5 to 15 nm.
 9. The method according to claim1, wherein a lateral size of the nanoparticles is from 26 to 50 μm. 10.The method according to claim 9, wherein the lateral size of thenanoparticles is from 35 to 45 μm.
 11. The method according to claim 1,wherein a nanoparticles concentration is from 0.01 to 12 wt. %.
 12. Themethod according to claim 11, wherein the nanoparticles concentration isfrom 2 to 8 wt. %.
 13. The method according to claim 12, wherein thenanoparticles concentration is from 4 to 7 wt. %.
 14. The methodaccording to claim 1, wherein the nanoparticles are multilayerednanoplatelets.
 15. The method according to claim 1, wherein thenanoparticles are selected from a group consisting of: graphitenanoplatelets, graphene, few layers graphene, TiO₂, ZnO₂, ZnO,Boron-nitride, copper, silica, montmorillonite, zeolite clipnoptilolite,wollastonite, mica, zeolite 4A, Al₂O₃, silicate, pumice and calciumoxide.
 16. The method according to claim 1, wherein the heat transferfluid includes a dispersing agent.
 17. The method according to claim 16,wherein the dispersing agent is a non-surface active polymer, asurfactant or a mixture thereof.
 18. The method according to claim 17,wherein the surfactant is cationic, anionic, amphoteric or non-ionic.19. The method according to claim 18, wherein the dispersing agent isselected from a group consisting of: polyvinnylpyrrolidone,polysaccharides, sulphated polysaccharides, linear alkylbenzenesulfonates, lignin sulfonates, di-alkyl sulfosuccinates, quaternaryammonium compounds and sodium stearate and a mixture thereof.
 20. Themethod according to claim 16, wherein a nanoparticlesconcentration/dispersing agent concentration ratio in weight is from 3to
 18. 21. The method according to claim 1, wherein the fluid medium isselected from a group consisting of: water, ethylene glycol, ethanol,oil, methanol, silicone, propylene glycol, alkylated aromatics, liquidGa, liquid In, liquid Sn, potassium formate and a mixture thereof. 22.The method according to claim 1, wherein the heat transfer fluid is inlaminar or turbulent regime flow.
 23. The method according to claim 1,wherein the item is metallic and is made of aluminum, steel, stainlesssteel, copper, iron, copper alloys, titanium, cobalt, metal composite ornickel.
 24. The method according to claim 1, wherein the metallic itemis a heat exchanger and the heat transfer is realized with the fluidbeing inside the heat exchanger.
 25. The method according to claim 1,wherein the metallic item is a metallic substrate and the heat transferis such that the heat transfer fluid is in direct contact with themetallic substrate.
 26. The method according to claim 25, wherein thecontact between the metallic substrate and the heat transfer fluid isrealized though jet impingement cooling, pool boiling, spray cooling ormicro-channel cooling.
 27. A method for the manufacture of a heattransfer fluid comprising: providing nanoparticles, a lateral size ofthe nanoparticles being between 26 and 50 μm, a thickness/lateral sizeratio of the nanoparticles being more than or equal to 0.00015 and lessthan 0.00044, wherein the nanoparticles do not include carbonnanotubess; providing a fluid medium; adjusting a nanoparticleconcentration in order to achieve percolation; and mixing thenanoparticles with the fluid medium.
 28. A heat transfer fluidcomprising: the heat transfer fluid recited in claim
 1. 29. A heattransfer fluid manufactured by the process of claim
 27. 30. A heattransfer fluid comprising: a fluid medium; and nanoparticles; a lateralsize of the nanoparticles being between 26 and 50 μm, athickness/lateral size ratio of the nanoparticles being more than orequal to 0.00015 and less than 0.00044, the nanoparticles not includingcarbon nanotubes.
 31. A method of heat transfer between a metallic ornon-metallic item comprising the step of: transferring heat between ametallic or non-metallic item and a heat transfer fluid including afluid medium and nanoparticles, a lateral size of the nanoparticlesbeing between 26 and 50 μm, a thickness/lateral size ratio of thenanoparticles being more than or equal to 0.00015 and less than 0.00044,wherein the nanoparticles do not include carbon nanotubes and areselected from a group consisting of: graphite nanoplatelets, graphene,few layers graphene, TiO₂, ZnO₂, ZnO, copper, silica, montmorillonite,zeolite clipnoptilolite, wollastonite, mica, zeolite 4A, Al₂O₃,silicate, pumice and calcium oxide.
 32. The method according to claim 1,wherein the thickness/lateral size ratio is 0.00025.